SNS - General Chemistry - Thermochemistry

Question 1

Thermodynamics

Laws of Thermodynamics

First Law

Answer 1

The total energy of a system and its surroundings is conserved. Energy is neither created nor destroyed Change in internal energy (∆E) = heat added (q) + work done on the system (w)

Question 2

Thermodynamics

Laws of Thermodynamics

Second Law

Answer 2

In any spontaneous process, the entropy of the universe increases. Processes that are spontaneous in one direction are not spontaneous in the other. Entropy (S) is the measure of the randomness of a system

Question 3

Thermodynamics Laws of Thermodynamics Third Law

Answer 3

The entropy of a pure crystalline substance at absolute zero (0K, -273⁰C) is zero

Question 4

Thermodynamics

Hess’s Law

Answer 4

States that, whether a reaction occurs in one step or in a series of steps, the change in enthalpy is an additive property. Changes in enthalpy (∆H) refers to the thermal energy gained or lost when a change takes place under constant pressure

Question 5

Thermodynamics

Spontaneity

Prediction

Gibbs Free Energy, G

Answer 5

A thermodynamic state function that combines the two factors that affect the spontaneity of a reaction - enthalpy and entropy. Represents the maximum amount of energy released by a process occuring at constant temperature and pressure that is available to perform useful work

For a change occuring at constant temperature and pressure, the formula is:

∆G = ∆H - T∆S

where S is entropy (randomness of the system), H is enthalpy (measure of the amount of heat).

Because the temperature is always positive (ie in K), the effects of the temperature and signs of ∆H and ∆S on ∆G can be predicted: -∆H, +∆S = spontaneous at all temperatures, +∆H, -∆S = non-spontaneous at all temperatures, -∆H, -∆S = spontaneous at low temperature, +∆H, +∆S = spontaneous at high temperature

Question 6

Thermodynamics Spontaneity Prediction Gibbs Free Energy, G ∆G>0

Answer 6

If ∆G is positive, the reaction is non-spontaneous but the reverse reaction is spontaneous

Question 7

Thermodynamics Spontaneity Prediction Gibbs Free Energy, G ∆G<0

Answer 7

If ∆G is negative, the reaction is spontaneous and forward

Question 8

Thermodynamics Spontaneity Prediction Gibbs Free Energy, G ∆G=0

Answer 8

If ∆G is zero, the reaction is at equilibrium

Question 9

Thermodynamics

Spontaneity

Prediction

Standard Free Energy Change

Answer 9

Change in free energy (∆G^o) associated with the formation of a substance from its elements, with all of these substances in their standard states

∆G^o = (sum ∆GFproduct) - (sum ∆GFreactants)

Question 10

Thermodynamics Spontaneity Prediction Standard Free Energy Change N₂ (g) + 3H₂ (g) → 2NH₃ (g) N₂ (g): ∆GF = 0 H₂ (g): ∆GF = 0 NH₃ (g): ∆GF = -16.66 kJ/mol

Answer 10

∆G = (2 x -16.66) - (3 x 0) + (1 x 0) = -33.32 kJ

Question 11

Thermochemistry

Systems

1. Closed
2. Isolated
3. Open

Answer 11

1. Can exchange energy but not matter with surroundings - eg steam radiator
2. Can exchange neither energy nor matter with the surroundings - eg insulated bomb reactor
3. Can exchange both energy and matter with surroundings - eg pot of boiling water

Question 12

Thermochemistry

Processes

Answer 12

A system undergoes a process when one or more of its properties changes - associated with a change of state

1. Isothermal process - occurs when the temperature of the system remains constant
2. Isobaric process - when the pressure of the system remains constant
3. Adiabatic process - when no heat exchange occurs

Question 13

Conversion Calories to Joules

Answer 13

1c = 4.184J

Question 14

Calculation of heat absorbed/released in a process

Answer 14

q = mc∆T

where m is the mass, c is the specific heat and ∆T is the change in temperature

Question 15

State Functions

Answer 15

When the state of a system changes, the values of the macroscopic properties (eg temperature, pressure, volume) also change. Properties whose magnitude depends only on the initial and final states of the system and not on the path of the change are called state functions.

Other examples include entropy, enthalpy, internal energy and free energy

Question 16

Standard Heat of Formation

Answer 16

∆H^of

Enthalpy change that would occur if one mole fo the compound was formed directly from its elements in their standard states

Question 17

Standard Heat of Reaction

Answer 17

∆H^orxn

= (sum of ∆H^of of products) - (sum of ∆H^of of reactants)

Hypothetical enthalpy change that would occur if the reaction were carried out under standard conditions

Question 18

Calculate ∆H for 3C (graphite) + 4H2 (g) → C3H8 (g)

C3H8 (g) + 5O2 (g) → 3CO2 (g) + 4H2O (l) ∆Ha = -2220.1 kJ

C (graphite) + O2 (g) → CO2 (g) ∆Hb = -393.5 kJ

H2 (g) + 0.5O2 (g) → H2O (l) ∆Hc = -285.8 kJ

Answer 18

1. 3C + 3O2 →3CO2 ∆Hb = (-393.5 x 3) = -1180.5 kJ
2. 4H2 + 2O2 →4H2O ∆Hc = (-285.8 x 4) = -1143.2 kJ
3. 3CO2 + 4H2O → C3H8 ∆Ha = +2220.1 kJ
4. ∆H for 3C (graphite) + 4H2 (g) → C3H8 (g) = 2220.1 - 1180.5 - 1143.2 = -103.6 kJ

Question 19

Bond Dissociation Energy

Answer 19

Average energy required to break bonds within one mole of gaseous molecules

Question 20

Calculate the enthalpy change for C (s) + 2H2 → CH4 (g)

Bond dissociation energies for H-H and C-H bonds are 436 and 415 kJ/mol respectively

∆Hf of C (g) is 715 kJ/mol

Answer 20

= 715 + (2 x 436) - (415 x 4)

= -73 kJ/mol

Question 21

Entropy

Answer 21

Measure of the randomness of a system. The greater the order of a system, the lower the entropy.

Meaured in units of energy/temperature, commonly J/K or cal/K

Question 22

1. Change in Entropy
2. Standard entropy change for a reaction

Answer 22

1. ∆S = q_rev / T

where ∆S is change in entropy, q_rev is the heat added to the system and T is the absolute temperature

2. ∆S^orxn = (sum of S^oproducts) - (sum of S^oreactants)